Indoor air quality has received immense attention since the early 1990s partially owing to the studies showing the level of pollutants in indoor environment is actually higher than in outdoor environment. In addition, people generally spend more than 80% of their time indoors, which contributes a higher risk from inhalation of pollutants than outdoors. In 1995, USEPA (US Environmental Protection Agency) identified that indoor air pollution was one of the top environmental risks. The indoor air pollutants may be categorized into three groups:
Gaseous pollutants and vapors that include volatile organic compounds (VOC) and inorganic compounds . . . ;
Particulate matters that include radioactive particles and environmental tobacco smokes . . . ; and
Biological pollutants that include bacteria, fungi, virus . . . .
Common methods of controlling indoor air pollution include controlling pollution sources, increasing the air exchange rate and using air purifiers. At present, the use of air purifiers becomes more popular to remove indoor air pollutants. Traditional air purifiers use filters to remove particulate matters or use sorption materials to adsorb gases or odors. However, these techniques only transfer the pollutants to another phase rather than removing the same, and additional disposal or handling steps are subsequently required. For instance, when an activated carbon filter is used to remove gaseous pollutants, the filter needs to be replaced quite frequently, which is costly and inconvenient to the consumers. An alternative remediation technology, which offers a number of advantages over conventional technologies, is the use of heterogeneous photocatalytic oxidation (PCO). PCO reaction can degrade various VOC pollutants into innocuous products such as CO2 and H2O with lower power consumption.
The principle of PCO is illustrated in FIG. 1. PCO commonly uses semiconductor photocatalysts and ultraviolet (UV) light to decompose organic compounds into benign and odorless constituents such as water vapor (H2O) and carbon dioxide (CO2). When the photocatalyst is irradiated by UV light, electron in the valence band (VB) is excited to a vacant conduction band (CB), producing a positive hole (h+) in the VB. The activation equation can be written as:TiO2+hv→h+e−  (1)
The positive holes (h+) and electrons (e) are powerful oxidizing and reducing agents, respectively. They efficiently produce OH. (hydroxyl radical) and O2− through the following reactions:Oxidation reaction: h++OH−→OH.  (2)Reduction reaction: e−+O2ads→O−2ads  (3)
OH. is a very powerful oxidizing substance. It is derived from the oxidation of adsorbed water or adsorbed OH−. When OH. encounters VOCs in the air, the following degradation reaction takes place:OH.+VOC+O2→nCO2+mH2O  (4)
The main limiting factors of PCO include the incomplete oxidation and slow reaction rate, both will result in various byproducts, some of which could be toxic. Studies have shown that there is an optimum humidity range for achieving the best PCO result. When the humidity is too low, there could be insufficient water molecules for generating hydroxyl radicals; when the humidity is too high, water vapor competes with TiO2 for adsorption sites, which decrease the rate of PCO. To improve reaction rate and minimize incomplete oxidation intermediates, humidity level at the surface of photocatalysts needs to be maintained in an optimum window. For instance, an optimum humidity range for removing toluene and formaldehyde by PCO could be about 1000 ppmv to 4000 ppmv, with TiO2 being the PCO catalyst.
WP97/09073A1 discloses a method and device for disinfecting an air stream containing microorganisms. Specifically, the air steam is provided with certain relative humidity e.g., greater than about 40% in view that PCO reaction requires that the relative humidity cannot be too low. However, WO97/09073 only mentioned generally the humidity of the air stream is controlled, without details regarding the implementation.
EP1980317A1 discloses a device for regaining deodoration function, esp., based on cold plasma. However, EP1980317A1 is not relating to photocatalytic oxidation of VOCs, and provides no disclosure of how the humidity of air flow is controlled.
WO2010/093796A1 discloses an air treatment device based on UV. Though WO2010/093796A1 mentions PCO reaction is dependent on relative humidity of the air, it only disclose a solution for designing the device according to certain relative humidity, or, adjust the parameters of the reaction system according to the given humidity, instead of changing the relative humidity of the air per se.